So, before Lee made his discovery physicists believed (or assumed, or conjectured, or theorized) that “the laws of physics were identical whether observed directly or as a mirror image." Lee and Yang proposed that this did not always hold. Their proposal was later confirmed by experiments.

So, what physicists believed before Lee, was not a law of nature. Lee challenged what his colleagues believed to be true.

But you are right all so called laws of physics are provisional.